The present invention relates generally to superconductors, and more particularly to an armored spring-core superconducting cable and method of construction.
Superconducting materials have the unique material property of having zero electrical resistance. In other words, superconducting materials can conduct electricity with no loss of energy. Superconducting materials exhibit this unique material property only when cooled below their respective critical temperature. Commercial applications of superconducting materials are generally limited to high temperature superconducting materials, which have a higher critical temperature than low temperature superconducting materials. High temperature superconducting materials, such as Bi-2212 and Bi-2223, are generally perovskites, a crystalline ceramic in which a combination of metal atoms are arranged in a crystal lattice containing planes of oxygen atoms. Each metal atom is chemically bonded to at least one oxygen atom, so that the overall material is an oxide ceramic. The superconducting material Bi-2212 has the chemical composition Bi2+x(Sr,Ca)3Cu2O8+d, and the superconducting material Bi-2223 has the chemical composition Bi2+x(Sr,Ca)4Cu3O10+d. The slight shifts x, d from integer composition refer to the empirical fact that the best properties are obtained when there is a slight excess of the indicated atom in the crystal.
In high-temperature superconductors, superconducting current flows in the planes of oxygen atoms within each crystal grain of the superconducting perovskite crystal. This leads to the necessity to achieve compaction of the grains within the conductor and alignment of the grains so that current can be transferred from grain to grain along the conductor.
The desired perovskite phase in high temperature superconductive materials, such as Bi-2212 and Bi-2223, is very difficult to form. If any of the fabrication conditions, such as the relative proportions of the several metal atoms, i.e., stoichiometry, the temperature cycle, and the oxygen content, vary from optimum conditions, other non-superconducting phases will be formed and the material will not be superconducting.
In many commercial applications, the superconducting material is packed into sheathes, i.e., tubes, of silver metal and the sheathes are formed into strands, either in the form of flat ribbons or round wires. The silver sheath, or silver matrix, contains the superconducting material while permitting oxygen to diffuse readily into and out of the silver sheath during the high-temperature heat treat when the reactions take place to form the superconducting phase. The best properties have been attained in superconducting strands that each contain many superconducting filaments of the superconducting material. A typical superconducting strand is formed by a multi-step procedure as follows.
Finely ground oxides or nitrates of the constituent metal atoms are mixed in the desired stoichiometry. When the oxides are used, they are mixed as finely ground solid powders. When the nitrates are used, each nitrate is dissolved to saturation in water, the saturated solutions are titrated in the desired ratio into a mixture solution, the solution mixture is evaporated or freeze dried or spray-evaporated to yield a nitrate powder mixture, and finally the nitrate powder mixture is baked at high temperature, typically at 600xc2x0 C. for 12 hours, to evolve the nitrate and leave a uniformly dispersed oxide mixture. The oxide mixture is then sintered by baking at a high temperature, typically xcx9c850xc2x0 C., at which the oxide mixture reacts to form the desired perovskite phase.
The sintered material, consisting largely of the desired perovskite phases, is ground to a fine powder and packed into a silver sheath. A number of packed silver sheathes are combined and then cold worked, by drawing or roll pressing, to form a metallurgically bonded strand. The strand is further drawn or rolled to reduce its cross-sectional area to the desired size. In the case of Bi-2223, a further heat treat is performed, typically xcx9c830xc2x0 C. for more than 24 hours, to promote the growth of long filamentary crystals of the perovskite phase within each filament of the strand. The strand is then cold worked again to reduce porosity and increase the alignment of the crystal grains within each the superconducting filament. If the strand is to be formed into a superconducting coil, such as used electrical or electromagnetic devices, it may be wound into a superconducting coil in this state of processing, with electrical insulation provided to insulate adjacent turns within the coil.
The strand or coil is then subjected to a final precision heat treatment cycle in a strictly controlled atmosphere. In the case of Bi-2212, this final heat treat is again at xcx9c830xc2x0 C. in an atmosphere containing 20% oxygen for 6 days, in which the metal oxides diffuse and react to form large aligned grains of perovskite phase within each filament of superconducting material. The control of temperature and oxygen within the strand or coil during this final heat treatment is critical to the performance of the superconducting strand. In the case of Bi-2223, this final heat treat is at lower temperature, typically xcx9c400xc2x0 C., and has the purpose of relieving stress within the superconducting crystals and the silver sheath.
Superconducting strands may be used in any suitable electrical or electromagnetic device, such as electric motors, generators, energy storage devices, transformers, magnetic bearings, high strength magnets used in magnetic resonance imaging, and the like. Superconducting coils generally comprise one or more superconducting strands that are wound around a core. Electricity flowing through the superconducting strands produce a magnetic field within the core. The strength of the magnetic field can be increased by increasing the number of times the superconducting strand is wrapped around the core and by increasing the current flowing through the superconducting strand. As will be discussed in greater detail below, the magnetic field produces a physical load on each individual superconducting wire, which is generally referred to by those skilled in the art as Lorentz stress.
Lorentz stresses produce an operational, or mechanical, load that acts to push the individual windings of superconducting strands away from the core. Lorentz stresses are produced by the magnetic field acting on the superconducting materials. The maximum Lorentz stress within a coil increases by the square of magnetic field strength produced by that coil. The operational load is transferred outward from each winding to each outwardly successive winding of superconducting strands. The magnetic field strength of the superconducting coil is generally limited by the operational loads that the outermost superconducting strands can support. Accordingly, the number of layers of superconducting strands that can be wound in a coil of superconducting strands is limited by the operational load that can be supported by a superconducting strand before its current capacity is degraded, which in turn limits the strength of the magnetic field that can be achieved in a conventional superconducting coil.
A technical disadvantage of conventional superconducting strands is that the silver sheathes that contain the superconducting material are soft and have low tensile strength. The operational loading from Lorentz stress must be supported by the silver sheathes. For this reason the silver is sometimes hardened to increase the strength of the strand. The hardening is typically achieved either by alloying with other metals with the silver, or by dispersing an insoluble material, typically an oxide, such as Al2O3, in the silver. The alloy or dispersion hardening may substantially increase the cost of the silver, and accordingly increase the cost of the superconducting strand. Even hardened silver has only moderate tensile strength compared to structural metals like stainless steel and Inconel. Accordingly the strength of the silver sheathes may limit the magnetic field that can be achieved by the superconducting coil. Also the strength of the silver sheathes may limit the use of superconducting coils in applications where shock or vibration are present. Most A.C. electrical devices must operate under conditions of moderate to severe shock and vibration. Accordingly the mechanical strength of the support sheathes may limit the use of the superconducting strands in such applications.
Another technical disadvantage of conventional superconducting strands is that the superconducting material contained within each superconducting strand is extremely brittle and cannot support a large strain. The current capacity depends critically upon the continuity and connectiveness of the grains of superconductor within the superconducting filaments. Compressive or bending stresses produce strain within the ceramic crystals of the superconducting filament, that can easily break the superconducting filaments, and disrupt the flow of electrical current. Accordingly, superconducting coils are generally fabricated using superconducting strands that are wound into the desired shape prior to the final heat treatment cycle. The coil is typically impregnated with epoxy to provide uniform support and transfer of stresses within the superconducting coil. Even with these provisions, however, the accumulation of Lorentz stress through a thick coil may limit the magnetic field that can be generated before the superconducting material is degraded in its current capacity.
Another technical disadvantage of conventional superconducting strands is that it is difficult to attain optimal conditions in the final heat treat for a superconducting coil containing many layers of windings. As discussed previously, the conditions of temperature and atmosphere surrounding the superconducting material must be controlled within very tight tolerances, typically on the order of xc2x12% oxygen pressure and xc2x12xc2x0 C. temperature, of the optimal conditions in order for the superconducting material to attain optimal current capacity. In a superconducting coil containing many layers of windings, the outer windings of the superconducting coil form a barrier that prevents the internal superconducting strands from being exposed to the optimum conditions. Accordingly, the access to oxygen may be inhibited in the internal windings of a superconducting coil during heat treatment, and the superconducting material in the internal windings may not attain its optimal current capacity. Accordingly, the internal windings of superconducting strands often limits the current carrying capability of the superconducting coil.
Another technical disadvantage of conventional fabrication processes is that it is difficult to provide adequate refrigeration of the superconducting coil. In operation, conventional superconducting coils are generally bathed in cryogenic liquids, such as liquid nitrogen or helium, to maintain the temperature of the superconducting coil well below the critical temperature at which it would lose its superconducting properties. The internal windings of superconducting strands are not directly cooled by the cryogenic liquid. Heat can be generated within a coil for a number of reasons. In particular, in A.C. applications it is common to wind a superconducting coil using a cable containing several superconducting strands. In such applications, the alternating currents induce non-superconducting currents between the superconducting strands of the cable. Such currents pass through the silver sheathes, and accordingly generate heat through ohmic resistance. If refrigeration is provided by immersing the impregnated coil in a bath of liquid cryogen, the heat generated within the coil must be conducted to the outer bounds of the coil in order to be removed. This produces a temperature gradient, so that the inner portions of the coil may be heated to a higher temperature than the outer portions of the coil. Quench occurs if the superconductive material increases in temperature above the critical temperature and is no longer superconducting. When this occurs, the superconductive material has an electrical resistance and the current flowing through the superconducting strand produces heat, which in turn heats the surrounding superconducting strands above the critical temperature. The coil has energy stored in its magnetic field. If a quench began at one location in a coil, and slowly propagated to surrounding regions, most of the energy stored in the magnetic field would be transformed into heat in the region near where the quench began, and it could reach a sufficiently high temperature, approximately 300xc2x0 C., wherein the epoxy impregnation would be irreversibly damaged. For this reason it is necessary to provide a means for forcing a quench to distribute throughout the coil once it begins at any location in the coil. Accordingly, the necessity for effectively controlling the quench of a superconducting coil restricts the number of windings in conventional superconducting coils, which limits the magnetic field strength that can be achieved in conventional superconducting coils.
A further disadvantage of conventional bath cooling of a superconducting coil is that the bath itself complicates the housing of the coil for many applications. In a motor or generator, for example, providing bath cooling of the coils would pose a major complication to the overall design of the device.
Accordingly, a need has arisen in the art for an improved superconducting cable. The present invention provides an armored spring-core superconducting cable and a method of construction that substantially eliminates or reduces problems associated with prior systems and methods.
In accordance with one embodiment of the present invention, an armored spring-core superconducting cable is provided. The armored spring-core superconducting cable comprises a spring-core, at least one superconducting strand wound onto the spring-core, and an armored shell that encases the superconducting strands. The spring-core preferably comprises a perforated tube that may have different cross sections, such as a circular or rectangular cross section. In an embodiment of a rectangular cross section spring-core, the spring-core includes a laminar spring that provides structural support to the tube. The superconducting strands may be fabricated in several different embodiments. For example, the superconducting strand may comprise a conventional superconducting strand with hardened silver matrix or a superconducting strand with pure silver matrix. The type of superconducting strand may be a round wire or a flat tape. In a particular embodiment, the armored spring-core superconducting cable also includes a conductive jacket formed outwardly of the armored shell. In another embodiment, an insulation layer is disposed between the spring-core and the superconducting strands, and the superconducting strands and the armored shell.
In accordance with another embodiment of the present invention, a superconducting coil for an electromagnetic device is provided. The superconducting coil comprises at least one armored spring-core superconducting cable wrapped around a core. The electromagnetic device using the superconducting coil may be a superconducting magnet, an electric motor, generator, energy storage device, or other suitable electromagnetic device.
In accordance with one implementation of the present invention, a method of constructing an armored spring-core superconducting cable is provided. In this implementation, a perforated tubular spring-core is provided. At least one superconducting strand is then wound onto the perforated tubular spring-core. The superconducting strands and the perforated tubular spring-core assembly are then encased within an armored shell to form the superconducting cable. In a particular implementation, the armored spring-core superconducting cable includes an insulation layer separating the superconducting strands from the spring-core on the inside, and the armored shell on the outside.
In accordance with one implementation of the present invention, a layer of low-resistance conductor, such as copper, may be attached to the outer surface of the armored shell, either as a surface coating on the armor shell or as a separate wire or strip that is wound along with the armored cable. The purpose of such conductor is to improve the stability of the magnet under quench conditions.
Technical advantages of the present invention include providing an armored spring-core superconducting cable that manages the various stresses within a superconducting coil. Internally generated stresses are managed by the spring-core and the external stresses are managed by the armored shell. Specifically, the spring-core preloads the superconducting strands within the armored spring-core superconducting cable and also deflects during operation to maintain the stress in the superconducting cables at a level far below the strain degradation limit of the superconducting material within the superconducting strands. In other words, the current carrying capacity of the superconducting material would be reduced if the strain in the superconducting material exceeded the strain degradation limit of the superconducting material. The armored shell protects the superconducting strands by transmitting the external stresses, such as the preload, Lorentz stresses from other armored spring-core superconducting cables, vibration, and shock, through the armored shell instead of through the superconducting strands. The stress management of the armored spring-core superconducting cable allows superconducting coils to be fabricated that can operate in stronger magnetic fields and with more tolerance of shock and vibration than conventional superconducting coils.
Another technical advantage of the present invention is that the armored spring-core superconducting cable allows precise control of the oxygen content in the superconducting coil during heat treatment of the superconducting strands, even in superconducting coils having many layers of windings. Heat treating the superconducting strands to produce a superconducting material within the superconducting strand requires precise control of the atmospheric conditions surrounding the superconducting strands. Purge gases can be circulated through the spring-core of the armored spring-core superconducting cable to maintain the optimal conditions throughout the superconducting coil during the heat treatment process. As a result, the superconducting material within each superconducting strand can be processed to achieve its optimum current capacity.
Another technical advantage of the present invention is that the temperature during the final heat treat can be controlled throughout the entire thickness of the superconducting coil, even for superconducting coils having many layers of windings. An electric current can be passed through the armored shell, spring-core, and/or the conductive jacket to resistively heat the superconducting coil uniformly throughout its cross-section. This provides a means to raise the temperature uniformly throughout the superconducting coil, without creating a temperature gradient between the outer and inner portions of the superconducting coil.
Another technical advantage of the present invention is that the armored shell, the spring-core, and the conductive jacket may be used to control the distribution of quench in the event that a quench originates anywhere in the superconducting coil. The armored shell, spring-core, and conductive jacket constitute electrically resistive current paths which are in parallel with the superconducting core of the cable and intimately coupled to it inductively. In the event that a region of the cable quenches, i.e., loses its superconducting property and becomes resistive, the current in the superconducting coil can be transferred through the armored shell, spring-core, and conductive jacket through inductive coupling. The current flowing in these parallel paths would produce resistive heating uniformly throughout the superconducting coil, driving the entire superconducting coil into a resistive state and preventing excessive heat from being deposited in the region where the quench began. Alternatively, upon detection that a quench had begun within the superconducting coil, an additional current could be applied to the armored shell, spring-core, and/or superconductive jacket to actively heat the entire superconducting coil into a resistive state and speed the process of energy dissipation.
Another technical advantage of the present invention is that the superconducting coil can be internally cooled by circulating cryogenic fluids through the spring-core of the armored spring-core superconducting cable. Because the cryogenic fluids are in intimate contact with all regions of each superconducting strand within the armored spring-core superconducting cable, the large heat of vaporization of the cryogen is available through local boiling to remove any heat that might be produced locally, for example because of A.C. losses in coils for A.C. applications, or because of a small region of a strand quenching in a high-field application. Depending upon the detailed design for a given application, this provision of enhanced cooling can actually make it impossible for a coil to quench, i.e., cryostability, so long as it is being adequately refrigerated. As a result, the superconducting coil can be fabricated with a greater number of windings of armored spring-core superconducting cables than is attainable in conventional superconducting coils. Accordingly, the superconducting coil can produce stronger magnetic fields as compared to conventional superconducting coils, and can be used in applications where shock or vibration are present or where there are sources of external heat.
Another technical advantage of the present innovation is that the oxygen required during the final heat treat of the coil is localized within the armored shell. It is generally necessary to provide an insulating material between adjacent windings in a superconducting coil. The insulation is installed as the coil is wound, and must be in place during the reaction bake. In many applications, it may be advantageous to form a conductive jacket onto the outside of the armored shell, in order to provide a parallel conducting path for coil current during quench. In both cases, if the insulation or the conductive jacket were exposed to oxygen at the temperature and for the duration of the final reaction bake, the properties of the insulator and/or the conductive jacket would be degraded by oxidation. By enclosing the superconducting strands within the armored shell and circulating the oxygen inside the armored shell, the insulation and the conductive jacket are effectively protected from the affects of the oxygen.
Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.